A method for controlling a hybrid drive of a vehicle is described in German Patent Application No. DE 102 02 531 A1. The hybrid drive includes an internal combustion engine and at least one electric machine as drive machines. The drive shafts of the drive machines may be connected to a drive train of the vehicle, and when there is a negative torque requirement (braking) on the drive train of the vehicle, the at least one electric machine is activated in the generator mode based on a characteristics map. The at least one electric machine is activated as a function of a setpoint braking torque MA,setpoint and a vehicle velocity v. The activation is performed by a regeneration strategy block which receives the necessary signals via an interface and which has function modules which generate the control signals for the at least one electric machine. According to the method described in German Patent Application No. DE 102 02 531 A1, a brake module selector is provided which optionally activates different operating modes for the braking as a function of signals present at the interface, the operating modes including strictly regenerative braking, combined regenerative and mechanical braking, and strictly mechanical braking.
Vehicles having a hybrid drive are characterized in that they have at least two different drive units. It is most common for a spark-ignition internal combustion engine to be combined with at least one electric machine, thus providing various possibilities for conserving energy. A parallel hybrid drive having one or two clutches represents one type of coupling of the two drive units. In the parallel hybrid drive, the internal combustion engine and the electric machine are provided in such a way that the entire drive torque results from the sum of the particular individual torques, except for the power requirement of the auxiliary units. A disconnect-type clutch is generally situated between the at least two drive units and the vehicle transmission.
The rotational speeds of the internal combustion engine and the at least one electric machine are generally linked. In this manner, for limited time periods which depend on the battery used for the at least one electric drive, hybrid driving may be performed in which the at least one battery for the at least one electric drive of the parallel hybrid drive is discharged. On the other hand, the parallel hybrid drive offers the possibility of charging the at least one battery, using the at least one electric machine, at a higher torque of the internal combustion engine than is actually required at the transmission input. For a drive train configuration having a clutch, the drag torque of the internal combustion engine which must be continually overcome would be in conflict with a strictly electric driving mode, but this could be avoided by using a second disconnect-type clutch between the internal combustion engine and the at least one electric drive. However, this is not favored due to the high level of additional mechanical and design complexity for an additional clutch between the internal combustion engine and the at least one electric drive.
Using a parallel hybrid drive, it is possible to convert kinetic energy to electrical energy during braking, and to feed the electrical energy to the at least one battery by recuperation. Because of the lack of a second disconnect-type clutch between the internal combustion engine and the at least one electric drive, or in situations in which a second disconnect-type clutch is present and the internal combustion engine should not be switched off, during a braking phase it is possible to feed electrical energy only as long as the rotational speed of the internal combustion engine, which is a function of the driving speed and the gear ratio, is greater than the minimum rotational speed of the internal combustion engine, so that for drive trains having two or more disconnect-type clutches, the disconnect-type clutch(es) may remain engaged. In that case, the braking torque applied by the drive train is composed of drag torque Mdrag of the internal combustion engine and the torque absorbed by the at least one electric machine operated in generator mode.